Peptides (polymers of amino acids) and peptide-like structures can influence a variety of physiological processes, such as the endocrine, neurological, immune and enzymatic processes, with high specificity and potency. As such, peptides are of critical importance to biomedical research and fields related thereto, and have found, or have potential use, in medicine, including the regulation of fertility, the control of pain, the stimulation of growth, the therapy of cancer, cardiovascular applications, treatment of connective tissue diseases, treatment of digestive disorders, treatment of mental illness, treatment of infections by pathogens, and the like.
The demand for peptides is therefore enormous and continues to increase. While natural sources can provide a great variety of peptides, often it is difficult to identify and isolate a particular peptide of interest in useful amounts. Accordingly, peptide synthesis by recombinant DNA or purely chemical means plays an important role.
Modern chemical synthesis of peptides has been summarized as follows: "All modern work in peptide synthesis employs the following approach: the amino group of one amino acid is first stabilized by the introduction of a protecting group R, the carboxyl group is modified so that it is capable of coupling with a second amino acid, the coupling is performed, and finally the group R is removed to produce a finished peptide. The difficulty lies in finding a suitable group R, which can be removed so gently that the peptide is not significantly attacked." Bergmann & Zervas, Berichte der Deutschen Chemischen Gesellschaft (1932) 65: 1192-1201.
For an "R" group to be suitable for use in peptide synthesis, it must satisfy a number of different criteria. It must suppress the nucleophilic reactivity of the .alpha.-amino group, either: (a) by draining the .alpha.-amino group's electron density away into an appropriate substituent or (b) concealing the group "behind a screen of gross steric hindrance." It should introduce no problems of its own. It should remain firmly in position while needed. It should be removable under conditions that do not adversely affect the rest of the structure being assembled. It should not affect the chiral integrity of nearby asymmetric centers. It should be orthogonal to other protecting groups that are being used.
A number of different "R" groups have been developed for use in chemical peptide synthesis, and include: the alkoxycarbonyl family of amino-protecting groups, such as the Benzyloxycarbonyl (Z) group, the t-Butoxycarbonyl (Boc) group, the 2-(4-Biphenylyl)isopropoxycarbonyl (Bpoc) group, the 9-Fluorenylmethoxycarbonyl(Fmoc) group, the 2,2,2-Trichloroethoxycarbonyl (Troc) group; the triphenylnethyl (trityl, Trt) group; the 2-Nitrophenylsulphenyl (Nps) group; the Dithiasuccinoyl (Dts) group; and the Diphenylphosphinyl (DPP) group.
A significant advance in the field of chemical peptide synthesis occurred in 1963 with Merrifield's development of the technique of solid phase peptide synthesis. Merrifield, J. American Chemical Soc. (1963) 85: 2149-2154. Methods of solid phase peptide synthesis are characterized by the iterative coupling of an .alpha.-amino protected amino acid monomer to a solid phase bound amino acid monomeric unit, in which the C-terminal amino acid of the desired peptide product is covalently bound to a solid phase, such as polystyrene. In addition to the .alpha.-amino protecting group, any reactive side chain functionalities that may be present on the individual amino acid monomeric units of the growing peptide chain are protected. As such, the "temporary .alpha.-amino protecting group must be orthogonal in reactivity to the side chain protecting groups." Since its development, different combinations of .alpha.-amino protecting groups and side chain protecting groups have been employed in solid phase peptide synthesis. For example, the Boc group has been employed as the .alpha.-amino protecting group in combination with benzyl groups as the side chain protecting groups. Of increasing prevalence is the use of the Fmoc group as the .alpha.-amino protecting group in combination with the Boc group, or a similar t-butyl based group, as the side chain protecting group. Although each of the above combinations of protecting groups have been successfully used in the chemical synthesis of a multitude of different properties, no protecting group has been yet been developed that is completely satisfactory.
Accordingly, there is continued interest in the identification of new .alpha.-amino protecting groups which are suitable for use in methods of chemical peptide synthesis. Because of its continued growth in popularity in the field of peptide synthesis, of particular interest is the development of new .alpha.-amino protecting groups suitable for use in solid phase peptide synthesis.
Relevant Literature
Jones, The Chemical Synthesis of Peptides (Oxford University Press) (1993) provides a review of both solution phase and solid phase peptide synthesis.